Sound is an important source of information to fish and other aquatic animals about their environment, because sound is transmitted through water very efficiently. Water is known to be an excellent conductor of sound waves because it is much denser than air, and its molecules are closer together. In addition, sound travels several times faster in water than in air. Moreover, sound travels farther in water than in air. The attenuation of sound over distance in water is much less than the attenuation of light.
In addition to sound waves, acoustical effects occur in water as follows: As one draws closer to a sound source, and the frequency of that sound gets lower, there is an actual outward movement of water that is generated along with the sound waves. This is analogous to sitting close to a speaker and feeling a faint burst of air when a low bass note is struck. In water, this movement of water is called the “near-field effect,” while standard sound waves (i.e., audible sounds) are known as the “far-field effect.” Fish sense near-field vibrations through their lateral lines, while detecting far-field sound waves with their inner ears, and these waves travel long distances in water.
While fish and other species may use all their senses to locate food, sounds and vibrations are key trigger mechanisms in the biological genetic response to attack prey, particularly those sounds and vibrations produced by schooling baitfish and distressed prey. Furthermore, sound is often of significant value in aquatic environmental conditions unsuitable for visual or olfactory chemical communication. For example, the effectiveness of visual stimuli in an underwater location can be limited by low light levels and/or turbidity (e.g., muddy water) or the like which severely attenuate the intensity of the light over distance. Chemical signals involving the olfactory sense of the fish propagate slowly, are non-directional and are easily diffused by water currents.
Fishermen have attempted to reproduce underwater sounds in order to attract fish and other species by means of lures that produce a sound or vibration. A number of rattling or vibrating lures have been produced which attempt to attract fish by electrically or mechanically generating and transmitting signals which simulate acoustics produced by baitfish. However, fish generally appear to produce acoustic signals which vary in signal frequency, periodicity and amplitude. Such complex signals are not readily reproduced by simple buzzers or other devices which generate signals of fixed frequency, duration and amplitude or which are varied in an arbitrary manner.
Further attempts have been made which involve using a frequency synthesizer to generate signals of varying frequency and broadcasting them underwater in order to influence the behavior of aquatic animals. In addition, underwater acoustical signals produced by actual species of aquatic animals have been recorded. For example, members of a particular species of baitfish may be isolated in a tank or other isolated environment, and signals produced underwater have been recorded by means of an underwater acoustical transducer. A hydrophone has also been used to record the sounds of one or more bass fish actually striking and consuming baitfish, such as a minnow or shad, and reproducing the recorded sounds underwater at a location where it is desired to attract bass.